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AS1302

Datasheet - A p p l i c a t i o n I n f o r m a t i o n

Flying Capacitor Selection

To ensure the required output current and avoid high peak currents the values of the flying capacitors CFLY1 and CFLY2

are very critical. A 220nF capacitor is sufficient for most applications. Dependent on the operation mode the AS1302

alternately charges and discharges the CFLY1/2 . While the ESR of the output capacitor produces a part of the output

voltage ripple, the ESR of the flying capacitors directly adds to the charge pump’s output source resistance. Therefore

low ESR capacitors, e.g. tantalum or ceramic, are recommended for the flying capacitors as well.

Due to different materials for ceramic capacitors the on the material depending temperature and voltage coefficients

have to be considered. The capacitance of a X7R ceramic capacitor is more stable than a Z5U or Y5V ceramic

capacitor over the whole temperature range from -40°C to +85°C. As an additional effect a Z5U or Y5V ceramic

capacitor will loose about the half of his nominal capacitance when the rated voltage is applied.

It is important to choose the ceramic capacitor according to the minimum available capacitance over the operating

voltage and the bias voltage. This information is stated in the datasheets of the capacitor manufacturer.

Layout Consideration

To achieve the best performance of the AS1302 a careful board layout is necessary to reduce the impact of the high

switching frequency and the high transient currents which are produced by the device. For a proper regulation under all

conditions a true ground plane and short connections to all external capacitors are needed.

Table 5. Recommended Flying Capacitors

Part Number

C

TC Code

Rated Voltage

Dimensions

Manufacturer

GRM188R71E224KA88

220nF

X7R

25V

0603

Murata

www.murata.com

GRM155R61A224KE19

220nF

X5R

10V

0402